1. Field of the Invention
A first aspect of the present invention relates to a non-aqueous ink that is suitable for use with an inkjet recording system, and specifically to a non-aqueous pigment ink that is highly effective to eliminate or reduce striking through and achieves high print density.
A second aspect of the invention relates to a non-aqueous ink that is suitable for use with an inkjet recording device, and in particular to a non-aqueous inkjet ink that is suitable for use with an ink circulation type inkjet recording device which performs printing by ejecting an ink circulating therein from an ink head.
2. Description of the Related Art
An inkjet recording system ejects a highly fluid inkjet ink as an ink particle from a very thin head nozzle to record an image on a sheet of printing paper, which is positioned to face the nozzle. Because of low noise and ability of high-speed printing, the inkjet recording systems are rapidly becoming widely used in recent years. As inks for use with the inkjet recording systems, so-called non-aqueous pigment inks, which are formed by finely dispersing a pigment in a non-water-soluble solvent, are known.
The non-aqueous pigment inks typically use a pigment dispersant for ensuring dispersion stability of the pigment. Therefore, after the ink has transferred onto a printing paper sheet, the solvent in the ink penetrates into the paper sheet, and the pigment penetrates into the paper sheet together with the solvent. This results in low print density and high level of striking through and is therefore problematic. The pigment dispersant has high affinity with the solvent and the pigment, and therefore the pigment tends to penetrate into the paper sheet when the solvent penetrates into the paper sheet. In order to address this problem, it is conceivable that use of an encapsulated pigment, wherein the pigment is surface treated with a polymer, such as one taught in Japanese Unexamined Patent Publication No. 2007-314651 (hereinafter, Patent Document 1), would allow keeping the pigment on the surface of the paper sheet, thereby eliminating or reducing the striking through.
On the other hand, as a non-aqueous inkjet ink (non-aqueous ink) suitable for use with an inkjet recording device that performs printing on a recording medium, such as a printing paper sheet, by ejecting an ink from a nozzle of an ink head, an ink containing a pigment, a pigment dispersant (polymer component) and a specific organic solvent, such as one taught in Japanese Unexamined Patent Publication No. 2004-2666 (hereinafter, Patent Document 2) is known. This type of ink is less likely to cause clogging of the nozzle and thus facilitates maintenance.
Among inkjet recording devices, there are ink circulation-type inkjet recording devices, in which the ink is always circulated during the printing state to cool the ink and remove dusts in the ink flow path. In the case of a line-type inkjet recording device which does not include a circulation-type head, defective ejection often influences the image quality. In contrast, in the case of a line-type inkjet recording device, which includes a circulation-type head and can circulate the ink through the path, the ink in the head is circulated and possibility of clogging of the head nozzle with an air bubble or foreign matter is low, and therefore the defective ejection is advantageously reduced.
However, temperature dependency of the ink viscosity of the non-aqueous inks tend to be higher than that of aqueous inks. Therefore, if an ink viscosity at room temperature is set at the center of an ejectable viscosity range, ink viscosities at lower temperatures tend to be out of the ejectable viscosity range, and it is necessary to warm up (heat) the ink to control the viscosity of the ink to be within the ejectable viscosity range.
The line-type inkjet recording device including the circulation-type head has the merit of lower possibility of defective ejection, as described above; however, has a demerit of long warm up time in a low temperature environment, since all the ink circulating in the inkjet recording device passes through the head and therefore a large amount of ink have to be warmed up. In order to address this problem, the present applicant has proposed, in Japanese Unexamined Patent Publication No. 2009-196208 (hereinafter, Patent Document 3), an inkjet recording device which changes the amount of circulating ink depending on the temperature of the circulating ink in the ink circulation path to reduce the time required for raising the temperature of the circulating ink to a prescribed temperature (which may hereinafter also be referred to as “time required for warming up”).